Small office and business environments commonly employ a multiplicity of work cells, each equipped with a telephone set and a computer. Two separate networks are usually employed for communication among the cells and between them and the outside world—a telephone network, connecting between the telephone sets and outside telephone lines, and a so-called local area network (LEN), connecting the computers among themselves and to outside network lines.
The term computer or personal computer will be understood to include a workstation or other data terminal equipment (DTE) or at least one digital device capable of inputting and outputting data, whereby each computer includes an interface for connection to a local area network (LAN), used for digital data transmission; any such device will also be referred to as a remote digital device. The term telephone set will be understood to include any device which can connect to a PSTN (Public Switched Telephone Network), using telephony band signals, such as fax machine, automatic answering machine or dial-up modem; any such device will also be referred to as a remote- or local telephone device.
Such an environment is depicted in FIGS. 1a and 1b, which show a typical small office/business configuration, requiring two separate and independent networks. FIG. 1a shows a telephony network 10 comprising a PABX (Private Automatic Branch Exchange) 11, connected via lines 12a, 12b, 12c and 12d to telephone devices 13a, 13b, 13c and 13d respectively. The telephone are of the POTS (Plain Old Telephone Service) type, requiring each of the connecting lines 12 to consist of a single pair of wires.
FIG. 1b shows a local area network (LAN) 15 for allowing communication between computers. Such a network comprises a hub (or switching hub) 16, connected via lines 17a, 17b, 17c and 17d to computers 18a, 18b, 18c and 18d respectively. Popular types of LANs are based on the IEEE802.3 Ethernet standard, using 10BaseT or 100BaseTX interfaces and employing, for each connecting line 17, two twisted pairs of wires—one pair for transmitting and one pair for receiving.
Installation and maintenance of two separate networks is complicated and expensive. It would therefore be advantageous, especially in new installations, to have a combined wiring network system that selves both telephony and data communication requirements.
One approach is to provide a LAN only, which serves for normal inter-computer communication, and make it serve also for telephony. One general method for this approach, in common usage today, utilizes so-called Voice-Over-Internet-Protocol (VoIP) techniques. By such techniques, known in the art, telephone signals are digitized and carried as data in any existing LAN. Systems employing such techniques are, however, complex and expensive, and the quality of the voice carried by currently available technology is low.
Another, opposite approach is to utilize an existing telephone infrastructure for simultaneously serving as both telephone and data networking. In this way, the task of establishing a new local area network in a home or other building is simplified, because there are no additional wires to install.
U.S. Pat. No. 4,766,402 to Crane teaches a way to form a LAN over two-wire telephone lines, but without the telephone service.
The concept of frequency division multiplexing (FDM) is well-known in the art, and provides a means of splitting the inherent bandwidth of a wire into a low-frequency band, capable of carrying an analog telephony signal, and a high-frequency band, capable of carrying data or other signals. Such a technique, sometimes referred to as ‘data over voice’, is described, for example, in U.S. Pat. Nos. 5,896,443, 4,807,225, 5,960,066, 4,672,605, 5,930,340, 5,025,443 and 4,924,492. It is also widely used in xDSL systems, primarily Asymmetric Digital Subscriber Loop (ADSL) systems.
A typical system employing FDM is illustrated in FIG. 2, which shows schematically a combined telephony/data network 20, providing in this case connections to two work cells by means of corresponding two cables 12a and 12b, each comprising a single twisted pair of wires. The lower part of the spectrum of cable 12a is isolated by Low Pass Filters (LPF) 22a and 22b, each connected to a respective end of the cable. Similarly, the higher part of the spectrum is isolated by respective High Pass Filters (HPF) 21a and 21b. The telephony network uses the lower spectrum part by connecting the telephone 13a and the PABX 11 to the respective LPFs. In order to use the higher part of the spectrum for data communication, telephone-line modems 23a and 23b are respectively connected to the HPFs 21a and 21b at both cable ends. Hub 16 connects to modem 23a, while, on the user side, modem 23b connects to computer 18a, thus offering connectivity between the computer and the hub. The spectrum of the other cable 12b is similarly split and cable 12b connects telephone set 13b to PABX 11 via LPFs 22c and 22d, while computer 18b connects to hub 16 via modem 23d, coupled to HPF 21d, and modem 23c, coupled to HPF 21c. Additional telephones 13 and computers 18 can be added in the same manner. This prior-art concept is disclosed in U.S. Pat. No. 4,785,448 to Reichert et al. (hereinafter referred to as “Reichert”) and U.S. Pat. No. 5,841,841 to Dodds et al. (hereinafter referred to as “Dodds”). Both Reichert and Dodds suggest a method and apparatus for applying frequency domain/division multiplexing (FDM) technique for residential telephone wiring, enabling simultaneously carrying telephone and data communication signals, as described above.
Network 20, employing an FDM method, typically requires two modems (such as 23a and 23b in FIG. 2) for each connected cell. Such modems are complex and expensive. In addition, the low communication quality of a typical telephone line, which was designed to carry low-frequency (telephony) signals only, limits both the data-rate and the distance of the data communication.
The concept of forming a phantom channel to serve as an additional path in a two wire-pairs communication system is known in the art of telephony, and disclosed in several patents, classified under U.S. Class 370/200. Commonly, such a phantom channel path is used to carry power to feed remote equipment or intermediate repeaters. In some prior-art systems, exemplified by U.S. Pat. Nos. 4,173,714, 3,975,594, 3,806,814, 6,026,078 and 4,937,811, the phantom channel is used to carry additional signals, such as metering and other auxiliary signals. Thus, all such systems use the phantom channel only as means for helping the communication service over the main channels. None of the mentioned prior-art uses the phantom channel for carrying an additional communication type of service, or for functionally combining two distinct networks.
It would thus be desirable to allow a data networking system to simultaneously also provide telephone service without any additional wiring.